This invention relates generally to valve actuators and deals more specifically with a manual override mechanism for a gate valve actuator, especially the hydraulic actuator of a subsea gate valve. The invention is directed additionally to a visual indicator device that provides a visual indication of the position of the gate.
Subsea gate valves are normally equipped with hydraulic actuators which control the position of the gate remotely from the surface. Typically, the gate is maintained in an open position by hydraulic fluid directed against a piston of the actuator. If an emergency occurs so as to cause a loss of fluid pressure in the actuator, the combined force of a spring and thrust of the valve stem moves the gate to a fail closed position. Conversely, some gates are held in a normally closed position by the fluid pressure and are moved to a fail open position in the event of a loss of hydraulic pressure. As still another alternative, the valve may be a fail-in-position type which is maintained in the position it occupies at the time of failure. With each type of valve, it is usually necessary or desirable to provide a manual override mechanism which permits the gate to be manually opened and closed.
In the past, a telescoping type override has been employed in order to minimize the size of the unit because space limitations take on primary importance in connection with subsea valves. This telescoping override arrangement is the closest known prior art to the present invention and includes a sleeve telescoped over a drive nut which is threaded onto the valve stem. A splined connection between the sleeve and drive nut translates rotation of the sleeve into rotation of the drive nut and consequent threaded advancement of the valve stem to open or close the gate. Rotation of the sleeve is facilitated by an adapter which is connected with the sleeve and which is formed to conveniently receive a wrench or handwheel or another type of tool. To provide for subsequent movement of the gate in an opposite direction in the event of a spring failure, a handwheel located on the opposite end of the valve may be turned to engage a secondary override mechanism. The secondary override acts directly against the gate to push it in a direction opposite the direction it is moved by the primary override.
Although this telescoping type of override mechanism has been generally satisfactory for subsea use, it has not been wholly without problems. The second override necessarily adds to the overall length of the valve, which is a significant drawback with subsea valves and with any other valve used in an environment where size is an important factor. Also, since the two override mechanisms are located on opposite ends of the valve body, the diver or other worker operating the override must have access to both ends of the valve and must know which handwheel to operate in a particular situation. The latter problem has been compounded because of the lack of any type of indicator to provide the diver with an indication of the gate position. The second override has the further disadvantage of requiring additional bearings, packing, seals, and other components which increase the cost and complexity of the valve.